you were wayyyyy overstocked and this is what caused your issues.
even if the filter could handle that bioload you would have been very high on nitrates and worse..TDS(total dissolved solids)
in essence the water was polluted and provided a bad environment rich in bacteria(the bad ones that lead to external and internal infections) ..you should have been doing large daily water changes to try and compensate.
even a 55 would have been overstocked in due time..you cannot keep 15 adult angels in a 55 you would need atleast 150 g tank for that many angels.
even if the filter could handle that bioload you would have been very high on nitrates and worse..TDS(total dissolved solids)
in essence the water was polluted and provided a bad environment rich in bacteria(the bad ones that lead to external and internal infections) ..you should have been doing large daily water changes to try and compensate.
even a 55 would have been overstocked in due time..you cannot keep 15 adult angels in a 55 you would need atleast 150 g tank for that many angels.
